An adjustable-depth ring assembly for mounting an electrical device in an electrical box that is accessible through a wall opening in a wall member disposed forward of the electrical box. In one embodiment, the assembly includes an extension ring, an abutment device, an electrical device, and a finish cover plate forming a pre-assembled unit. The unit is free of any connection with the box cover plate and the electrical box prior to field installation of the unit to connect the unit to the box cover plate and the electrical box. In another embodiment, the assembly includes an abutment device on the extension ring movable from a non-abutting position to an abutting position projecting outward beyond a perimeter of the extension ring for abutting a front surface of a wall member when the extension ring is moved to an installed position.